The present invention relates to direct current machines, and more specifically to homopolar machines.
Homopolar machines are operated by direct current (DC) and are simple in design principle. They have been under development for consideration in ship propulsion applications because of their high efficiency, compact size, low weight, and reduced acoustic signature relative to all other motors.
As is well known in the art, a homopolar machine includes four major components: armature; stator; field coils; and flux return. The armature is connected to the machine's shaft and may also be referred to as the rotor. The armature typically includes a series of concentric, copper cylinders and is free to rotate in a direction at right angles to the magnetic field lines produced by the field coils. When a voltage is applied across the armature in the direction of the shaft, electric current flows parallel to the shaft. The current and magnetic field interaction (I×B) results in torque generation and rotation, thus producing a motor. In contrast, if the armature is driven externally for a generator application, the interaction of the armature rotating at right angles to the magnetic field lines generates a voltage and electric current.
In both the motor and generator scenarios, current flows along the armature and to the stationary stator via sliding electrical contacts referred to herein as current collectors. The current collectors may also be commonly referred to as brushes, and may take the form of many materials. Such materials include but are not limited to flexible fibrous copper, flexible copper strips, or as common to most DC motors, rigid material made from graphite or silver-graphite.
The field coils are typically circumferentially continuous in geometry and aligned on the same central axis with respect to each other. A homopolar machine always cuts (or crosses as it rotates) magnetic flux lines of a magnetic field in the same direction due to the interacting armature and shaft iron being aligned on the same axis. This means that any point on the rotor always sees the same magnetic field as it rotates and no difference in magnetic flux or multiple magnetic poles are encountered by conductive elements of the armature as it rotates. Hence the nomenclature “homopolar machine”.
The flux return is typically comprised of a highly magnetically permeable material such as iron or steel. The flux return is designed primarily to limit the undesirable stray magnetic field that radiates from the machine, and therefore, it typically takes the form of a structural housing that surrounds the motor. In addition, the flux return may also be designed to help direct the magnetic field lines produced by the field coils into the armature interaction region to improve the machine's flux utilization.
Although conventional rotating machines are in wide use, most have inherent disadvantages associated with mechanical bearing wear. For example, large machines such as generators connected to turbines, as well as motors used for propulsion, use thrust bearings to react the induced axial mechanical forces on the shaft and to maintain a stable shaft axial position. The mechanical thrust that is induced on the shaft induces an axial thrust on the thrust bearing, which causes thrust bearing wear and noise. The mechanical thrust induced on the shaft may be from many sources. For example, in a generator application the induced thrust is caused by the axial pressure differential across a connected turbine, and in a propulsion motor application the induced thrust is caused by the thrust exerted by the connected propulsor. If a homopolar machine was used in these applications, it would be prone to this same type of problem. Ships, submarines and airplanes all experience thrust bearing forces within their propulsion systems. The induced axial mechanical forces are also referred to herein as thrust bearing loads.
As thrust bearing loads increase, wear and tear and associated noise increase at a rate on the order of pressure cubed (i.e., p3 where p is pressure due to the thrust bearing load). This means that a reduction of pressure on the shaft by two fold translates into a reduction of associated wear by eight fold. Such a reduction of pressure on the shaft would result in a thrust bearing lifetime of eight times its original lifetime. Thus, there is a need for a method and/or apparatus that helps to reduce thrust bearing loads in homopolar machines.
A unique disadvantage of homopolar machines is that they tend to have lower reliability in comparison to standard DC motors. Specifically, homopolar machines use current collectors to transfer current between each rotating armature turn and each stationary stator turn. One limitation of the utility of homopolar machines is the heavy dependence on current collectors that are potentially unreliable, a large source of efficiency loss, and maintenance problems.
An important factor in a current collector's performance is its contact pressure with the armature. Performance is measured in terms of current collector wear and current carrying capability. Maintaining an ideal contact pressure is difficult because in a homopolar machine the current collectors must be in the magnetic field zone where the collectors are subject to bending and torque. Homopolar machines have been designed with mechanisms that help to maintain an ideal contact pressure, but these result in size, weight, and cost penalties, and introduce new sources of reliability problems. Thus, there is a need for a method and/or apparatus that helps to reduce the wear and tear of current collectors in homopolar machines.